Hitherto, a modulation circuit, a demodulation circuit, and a wireless unit designed to be able to flexibly compatible with a plurality of modulation-demodulation systems have been known as a wireless terminal compatible with a plurality of communication systems (for example, refer to patent document 1).
FIG. 16 is a block diagram showing a transmitter as a wireless unit compatible with a plurality of communication systems in a related art described in patent document 1. In FIG. 16, the transmitter includes a modulation circuit 1100 for providing various types of modulation functions, an LPF 1117 for limiting the band of a baseband signal (made up of an I signal and a Q signal) output from the modulation circuit 1100, a quadrature modulator 1118 for converting the baseband signal whose band is limited into an IF signal, an up converter 1119 for converting the frequency of the IF signal provided by the quadrature modulator 1118 into a transmission frequency based on a local signal from a local oscillator 1120, a power amplifier 1121 for amplifying the power of the transmission signal whose frequency is converted into the transmission frequency, and a transmission antenna 1122.
The modulation circuit 1100 includes a QPSK modulation circuit 1102, an 8PSK modulation circuit 1103, a 64QAM modulation circuit 1104, a spread modulation circuit 1107, a first OFDM modulation circuit 1110, a second OFDM modulation circuit 1111, switches 1101, 1105, 1106, 1108, 1109, and 1112 for switching the modulation circuit connection relationship, a modulation control circuit 1115 for controlling the switches to switch the modulation circuits, thereby making it possible to be flexibly compatible with a plurality of communication systems, a power control circuit 1114 for controlling power supply to the modulation circuits, and a D/A converter 1113.
Development of a communication system in preparation for next-generation mobile communication service (fourth-generation mobile communication service) is started and an art of using VSF-OFCDM system in the down direction from a base station to the user and using MC (multicarrier)/DS-CDMA system with two carriers in the up direction is proposed (refer to non-patent document 1).    Patent document 1: JP-A-2003-318999 (FIG. 1)    Non-patent document 1: “Nikkei Electronics” 2002 Oct. 7 issue, p 129-p 138